Classic visual aftereffects have provided a powerful tool for psychophysically characterizing the receptive field properties of neural units involved in coding local image features, providing a link between physiologically specified receptive fields of cells in V 1 and perception. However, the application of aftereffects has been limited primarily to studying low-level coding of local features because the aftereffects bore signatures of early processing (e.g., retinotopy, narrow tunings for orientation and spatial frequency). Recently, Suzuki and colleagues and others have reported that under specific temporal conditions (brief adaptation probed with a brief test pattern following a brief adapt-to-test interval), aftereffects can be induced for global shape attributes such as aspect ratio, skew, taper, curvature, and convexity. These brief shape aftereffects bear signatures of high-level coding (e.g., tolerance for position, scale, and/or defining surface features). Brief shape aftereffects are thus promising as a tool for characterizing a processing stage in which global configurations of oriented contours are systematically coded. Detailed characterization of this "mid-level" visual coding will expand understanding of how visual pattern processing evolves from the coding of local image features to the coding of various intermediate geometric features, and eventually to the coding of representations of meaningful objects. The oal of this proposal is three-fold. In Aim 1, the temporal requirements for producing global shape aftereffects (as opposed to local image aftereffects) will be carefully specified to refine brief shape aftereffects as a tool for probing global shape coding. In Aim 2, brief shape aftereffects will be used to psychophysically characterize the receptive field properties and the nature of population coding operating at the stage of coding global shape attributes. In a real-world environment where objects routinely overlap, position-tolerant coding of global shape attributes cannot be considered without reference to an attentional selection mechanism. Consistent with this idea, brief shape aftereffects have been shown to be sensitive to attention. Thus, in Aim 3, the experiments are designed to investigate how observers' attention and image parameters interact to select individual shapes for global shape coding. Thus, the proposed three-fold investigation will provide a thorough psychophysical characterization of the receptive field and coding properties of the mid- to high-level shape representation whose activation depends on both the physical properties of the stimuli and attention. The results will begin to provide a link between the physiologically specified receptive fields in the higher cortical visual areas (e.g., in the ventral "pattern processing stream" such as V4 and IT) and perception of global shape attributes.